Clostridium perfringens type B, C, and D isolates have significant medical, veterinary, and biodefense importance. Several toxins (e.g. select agent "B" list epsilon toxin and beta toxin) expressed by type B-D isolates are encoded by genes present on large plasmids. The long-term goal of this project is to fully understand how those little-studied plasmids contribute to the virulence of type B-D isolates. To start progressing towards this goal, the following specific aims will be pursued: 1) constructing isogenic single and double toxin knock-out mutants in type B-D backgrounds using insertional mutagenesis approaches, 2) comparing the virulence of these newly-constructed isogenic mutants against their parents (and complemented mutants strains); this will be accomplished using in vivo and in vitro approaches that will evaluate enteric virulence (intestinal loop models), systemic virulence (intravenous injections of culture supernatants} and effects of an intraduodenal challenge mimicking the entire disease spectrum (i.e., both enteric and systemic disease), 3) using Aim #1 toxin mutants, which will carry virulence plasmids tagged with antibiotic resistance determinants, in mixed mating experiments to evaluate whether type B-D virulence plasmids can transfer between C. perfringens isolates via conjugation, 4) conducting phenotypic/genotypic analyses to evaluate the diversity of these isolates; these studies will involve examining type B-D isolates to determine how much beta- and/or epsilon-toxin (as appropriate) they produce, testing whether those toxin expression differences are related to promoter differences, determining if some type B-D isolates produce beta- or epsilon-toxin variants (as appropriate) with altered biologic activities, and examining the diversity of type B-D plasmid genomes using pulsed-field gel electrophoresis and microarray approaches, and 5)investigating non-toxin virulence plasmid functions by insertional inactivation approaches; if Aim #3 confirms that type B-D virulence plasmids can transfer via conjugation, Aim #5 will initially target putative DNA transfer genes on these plasmids. These studies are expected to provide critical information for developing improved vaccines/therapeutics against type B-D infections and for developing molecular assays to subtype these isolates, as necessary for forensic investigations in the event that type B-D isolates are deliberately released during a bioterrorism event.